The present invention relates to fuser apparatus for use in electrostatographic printing and, more particularly, to an improved pressure member for fixing toner to a receiver.
Heat-softenable toners are widely used in imaging methods such as electrostatography, wherein electrically charged toner is deposited imagewise on a dielectric or photoconductive element bearing an electrostatic latent image. Most often in such methods, the toner is then transferred to a surface of another substrate, for example, a receiver sheet comprising paper or a transparent film, where it is fixed in place to yield the final desired toner image.
Heat-softenable toners comprising, for example., thermoplastic polymeric binders, are generally fixed to the receiver sheet by applying heat to the receiver sheet surface to soften the toner transferred to it, and then allowing or causing the toner to cool.
One such well-known fusing method comprises passing the toner-bearing receiver sheet through the nip formed by a pair of opposing rolls, at least one of which, usually referred to as a fuser roll, is heated and brought into contact with the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roll, usually referred to as a pressure roll, serves to press the receiver sheet into contact with the fuser roll. In some other fusing methods, the apparatus is varied so that the fuser roll and/or the pressure roll take the form of a flat plate or belt. The description herein, while generally directed to a generally cylindrical fuser roll in combination with a generally cylindrical pressure roll, is not limited to fusing systems having members with those configurations. For that reason, the more general terms xe2x80x9cfuser memberxe2x80x9d and xe2x80x9cpressure memberxe2x80x9d are preferably employed.
In FIG. 1 is schematically depicted a fuser apparatus that includes a fuser roll 20 and a pressure roll 28 that form a nip 30. A supply of offset preventing oil 33 is provided in an oil reservoir 34. Particulate imaging material 40 disposed on a receiver 42 is fused onto receiver 42 at the nip 30 by the application of heat and pressure. As shown, a heating lamp 44 is connected to a control circuit 46. Alternatively, heat may be provided externally by a heated roll (not shown) riding along the fuser roll 20. The external heating means may supplant or merely assist the heating lamp 44. In some instances, the particulate imaging material 40 may be fixed onto receiver 42 by the application of pressure alone.
FIG. 1 also shows a wicking device 32 in the form of a wick 36, which absorbs the offset preventing oil 33 is contacted by a metering roll 48. Intermediate between fuser roll 20 and intermediate roll 48 is a donor roll 50, which delivers offset preventing oil 33 to the particulate imaging material 40 on receiver 42.
A fuser member usually comprises a rigid support covered with a resilient material, commonly referred to as a xe2x80x9cbase cushion layer.xe2x80x9d The resilient base cushion layer and the amount of pressure exerted by the pressure member serve to establish the area of contact of the fuser member with the toner-bearing surface of the receiver sheet as it passes through the nip of the fuser member and pressure members. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser member. The degree of hardness, often referred to as xe2x80x9cstorage modulusxe2x80x9d, and the stability thereof, of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
In some previous fusing systems, it has been found advantageous to vary the pressure exerted by the pressure member against the receiver sheet and fuser member. This variation in pressure can be provided, for example in a fusing system having a pressure roll and a fuser roll, by slightly modifying the shape of the pressure roll. The variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rolls, can be established by, for example, continuously varying the overall diameter of the pressure roll along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, in order to give the pressure roll a sort of xe2x80x9cbow tiexe2x80x9d or xe2x80x9chourglassxe2x80x9d shape. This will cause the pair of rolls to exert more pressure on the receiver sheet in the nip in the areas near the ends of the rolls than in the area about the midpoint of the rolls. This gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip. Over time, however, the fuser roll begins to permanently deform to conform to the shape of the pressure roll and the gradient of pressure is reduced or lost, along with its attendant benefits. It has been found that permanent deformation, often referred to as xe2x80x9ccreepxe2x80x9d, of the base cushion layer of the fuser member is the greatest contributor to this problem.
Particulate inorganic fillers have been added to base cushion layers to improve mechanical strength and thermal conductivity. High thermal conductivity is advantageous when the fuser roll is heated by an internal heater, enabling the heat to be efficiently and quickly transmitted toward the outer surface of the fuser roll and the toner on the receiver sheet that is intended to be contacted and fused. High thermal conductivity is not so important when the roll is intended to be heated by an external heat source.
Polyfluorocarbon elastomers such as vinylidene fluoride-hexafluoropropylene copolymers are tough, wear resistant, flexible elastomers that have excellent high temperature resistance but relatively high surface energies, which compromises toner release. Fluorocarbon resins such as polytetrafluoroethylene (PTFE) or fluorinated ethylenepropylene (FEP) are fluorocarbon plastics that have excellent release characteristics due to very low surface energy. Fluorocarbon resins are, however, less flexible and elastic than fluorocarbon elastomers and are therefore not suitable alone as the surface of the fuser roll.
Fuser rolls having layers formed from compositions comprising polyfluorocarbon elastomers and/or fluorocarbon resins are disclosed in, for example, U.S. Pat. Nos. 4,568,275; 5,253,027; 5,599,631; 4,853,737; 5,582,917; and 5,547,759, the disclosures of which are incorporated herein by reference. U.S. Pat. No. 5,595,823, the disclosure of which is incorporated herein by reference, discloses toner fusing members which have a substrate coated with a fluorocarbon random copolymer containing aluminum oxide. Although these toner fusing members have proved effective and have desirable thermal conductivity, they have a problem in that there can be toner contamination. The advantage of using the cured fluorocarbon thermoplastic random copolymer compositions is that they are effective for use with toner release agents that typically include silicone.
Polysiloxane elastomers have relatively high surface energy and relatively low mechanical strength, but are adequately flexible and elastic and can produce high quality fused images. After a period of use, however, the self-release property of the roll degrades, and offset begins to occur. Application of a polysiloxane fluid during roller use enhances the ability of the roller to release toner, but shortens roller life due to oil absorption. Oiled portions tend to swell and wear and degrade faster.
One type of material that has been widely employed in the past to form a resilient base cushion layer for fuser rolls is a condensation-crosslinked siloxane elastomer. Disclosure of filled condensation-cured poly(dimethylsiloxane) xe2x80x9cPDMSxe2x80x99 elastomers for fuser rolls can be found, for example, in U.S. Pat. Nos. 4,373,239; 4,430,406; and 4,518,655. A widely used siloxane elastomer is a condensation-crosslinked PDMS elastomer, which contains about 32-37 volume percent aluminum oxide filler and about 2-6 volume percent iron oxide filler, and is sold under the trade name, EC4952, by the Emerson Cumming Co., U.S.A. Despite some serious stability problems developing over time, materials such as EC4952 initially provide very suitable resilience, hardness, and thermal conductivity for fuser roll cushion layers.
The present invention is directed to an improved pressure member for use in fixing toner to a receiver. The pressure member comprises a support, an intermediate layer disposed on the support, and an outermost layer formed from a cured composition comprising a fluorocarbon thermoplastic random copolymer, a curing agent, a particulate filler containing zinc oxide, and a curable aminosiloxane, wherein the fluorocarbon thermoplastic random copolymer has subunits of:
xe2x80x94(CH2CF2)xxe2x80x94, xe2x80x94(CF2CF(CF3))yxe2x80x94, and xe2x80x94(CF2CF2)zxe2x80x94, and
x is from 1 to 40 or 60 to 80 mole percent,
y is from 10 to 89 mole percent,
z is from 10 to 89 mole percent, and
x+y+z equals 100 mole percent.